[CJL5585]

Make an open ring out of wood or metal. Support it horizontally and attach your film to the ring. Push the film down and fill it with water; the weight of the water will cause the film to form a catenary curve which is probably close enough to what you need. Now the more awkward part; fiberglas the bottom side of the film so it retains the shape when you empty out the water.

Good idea Geof. Thanks.